1. Field of the Invention
The present invention is directed to audio conferencing systems, and more particularly to a method of reducing acoustic coupling and howling effects in a full duplex audio conferencing system using beamforming.
2. Description of the Related Art
Beamforming technology (also known as microphone array technology) can be used for spatial directivity of sound pickup in a conferencing system to enhance the quality of near-end speech. For such systems that perform full-duplex communications, acoustic echo cancellation presents numerous challenges.
One problem that is well known in the art relates to combining acoustic echo cancellation with beamforming (see M. Brandstein and D. Ward, “Microphone Arrays. Signal Processing Techniques and Applications”. Springer Verlag, 2001, and H. Buchner, W. Herbordt, W. Kellermann, “An Efficient Combination of Multi-Channel Acoustic Echo Cancellation With a Beamforming Microphone Array”, Proc. Int. Workshop on Hands-Free Speech Communication (HSC), pp. 55-58, Kyoto, Japan, April, 2001). One approach is to perform acoustic echo cancellation on all the microphone signals in parallel, which is computationally intensive. A second approach is to perform acoustic echo cancellation on the spatially filtered signal at the output of the beamformer.
In the approach where acoustic echo cancellation is performed on the spatially filtered signal at the output of the beamformer, it is clearly desirable that the beamformer presents good loudspeaker-to-beam coupling characteristics in order to minimize the amount of echo that has to be cancelled. At the same time, it is also desirable that the beamformer has good directivity in its look direction to provide enhanced speech quality based on spatial directivity. In practice, however, it is impossible to meet both of these requirements with the same beamformer design as there typically is a trade off between the beamformer's directivity and response to the loudspeaker signal, as discussed below.
A number of design requirements must be met to ensure good coupling characteristics. Firstly, the beamformer must be robust to uncorrelated perturbations of the microphone signals. The reason for this is that in a conference system, the physical coupling between the loudspeaker and the microphones is subject to variations caused by the loudspeaker and other structural vibrations or leakage. Therefore, a high sensitivity of the beamformer to such variations may result in excessive acoustic coupling between the loudspeaker and the output of the beam. This, in turn, affects the performance of the acoustic echo cancellation, resulting in echo bursts as well as howling effects due to positive loop gain. It should be noted that the sensitivity problem applies to both structures described above for combining acoustic echo cancellation with beamforming. Indeed, even if acoustic echo cancellation is performed on the microphone signals in parallel, the residual echo signals are not free of amplitude and phase variations due to structural coupling (vibrations and acoustical leaks). Many of these variations will remain on the microphone signals after the individual echo cancellation. When combined in the beamformer they may result in a large error signal if the beamformer is sensitive to the variations described above. The second design requirement that must be met to ensure good coupling characteristics is that the beamformer should also provide strong attenuation of the loudspeaker signal (which can be seen as an interference signal) in order to achieve better echo cancellation performance in the conferencing system.